Process and unit for production of polycrystalline silicon film

ABSTRACT

A process for making a polycrystalline silicon film includes forming, on a glass substrate, an amorphous silicon film having a first region and a second region that contacts the first region, forming a first polycrystalline portion by irradiating the first region of the amorphous silicon film with laser light having a wavelength not less than 390 nm and not more than 640 nm and forming a second polycrystalline portion that contacts the first polycrystalline portion by irradiating the second region and the portion of the region of the first polycrystalline portion that contacts the second region of the amorphous silicon film with the laser light.

TECHNICAL FIELD

The present invention relates to a process and a unit for production ofa polycrystalline silicon film as well as a semiconductor device and aprocess for the same, in particular, to a process and a unit forproduction of a polycrystalline silicon film with excellentcrystallinity, for making a thin film transistor, having high carriermobility as a semiconductor device, and a process for the making thesemiconductor device and in which the polycrystalline silicon film isused.

BACKGROUND ART

At present, the pixel part of a liquid crystal panel forms imagesthrough switching of thin film transistors fabricated with an amorphousor polycrystal silicon film on a substrate of glass or synthesizedquartz. In the case that it becomes possible to simultaneously form adriver circuit (at present mainly installed independently outside of thepanel) to drive pixel transistors on this panel, there would be greatmerit with respect to production cost, reliability, and the like, of theliquid crystal panel. At present, however, the crystallinity of thesilicon film that forms an active layer of transistors is poor and,therefore, the performance of thin film transistors, represented bymobility, is low and it is difficult to fabricate an integrated circuitwherein high speed and high performance are required. As a method ofimproving the crystallinity of the silicon film for the purpose ofimplementing a thin film transistor with a high mobility, heat treatmentis, in general, carried out by using a laser.

The relationships between the crystallinity of a silicon film and themobility of a thin film transistor are described in the following. Asilicon film gained through laser heat treatment is, in general,polycrystal. Crystal defects locate in crystal grain boundaries of thepolycrystal silicon and they block the carrier movement in the activelayer of the thin film transistors. Accordingly, the number of timesthat the carriers cross the crystal grain boundaries while movingthrough the active layer becomes less and the concentration of crystaldefects becomes lower in order to enhance the mobility of the thin filmtransistors. The purpose of the laser heat treatment is to form apolycrystal silicon film of which the crystal grain diameters are largeand wherein there are fewer crystal defects in the crystal grainboundaries.

FIGS. 18 to 20 are cross sectional views for describing a process for apolycrystal silicon film according to a prior art. First, referring toFIG. 18, a silicon oxide film 32 is formed on a glass substrate 31 by,for example, carrying out CVD (chemical vapor deposition) on a glasssubstrate. An amorphous silicon film 33 is formed on a silicon oxidefilm 32 by means of, for example, CVD.

Referring to FIG. 19, an excimer laser (KrF (wavelength: 248 nm))irradiates an amorphous silicon film 33 in the direction shown by arrow335. Thereby, the portion irradiated by the excimer laser melts. Afterthat, as the temperature becomes lower, the melted silicon iscrystallized so as to form a polycrystal silicon film 334.

Referring to FIG. 20, polycrystal silicon film 334 is patterned so thatpolycrystal silicon film 334 only remains in portions. Next, a siliconoxide film and a metal film (low resistance metal film such as Ta, Cr orAl) are formed on polycrystal silicon film 334. Gate insulating films 36a and 36 b, as well as gate electrodes 37 a and 37 b, are formed bypatterning the metal film and the silicon oxide film. Thereby, activeregions 39 a and 39 b are formed. Next, source and drain regions areformed in a self-aligned manner by means of an ion doping method byusing gate electrodes 37 a and 37 b as a mask. Thereby, the thin filmtransistors shown in FIG. 20 are completed.

According to a conventional method, as shown in FIG. 19, an amorphoussilicon film is polycrystallized by using an excimer laser and,therefore, the mobility of the carriers is low in the transistors formedon the polycrystal silicon film. As a result, a high speed of operationof the transistors is difficult so that it is difficult to achieve ahigh response of the liquid crystal display device.

In addition, Reference 1 (T. Ogawa, et al., “Thin Film Transistors ofPolysilicon Recrystallized by the Second Harmonics of a Q-Switched Nd:YAG Laser” EuroDisplay '99 The 19^(th) International Display ResearchConference Late-news papers Sep. 6-9, 1999 Berlin, Germany) discloses anamorphous silicon film made polycrystalline by using, for example, thesecond harmonic of an Nd: YAG laser as a laser light and a thin filmtransistor formed by using this polycrystalline film wherein themobility is increased. However, since the output of the second harmonicof the YAG laser is small, an amorphous silicon film having only a smallarea can be made polycrystalline. Therefore, the manufacturing of apolycrystalline silicon film for manufacturing a liquid crystal displayhaving a large area is difficult.

Therefore, this invention is provided in order to solve the abovedescribed problems.

One purpose of this invention is to provide a process and a unit formanufacturing a polycrystal silicon film that is suitable in thefabrication of a thin film transistor of high performance and that has alarge area.

In addition, another purpose of this invention is to provide a thin filmtransistor of high performance and a process for the same.

DISCLOSURE OF THE INVENTION

A process for a polycrystal silicon film according to this invention isprovided with the following steps:

(1) the step of forming an amorphous silicon film having a first regionand a second region that contacts this first region on a substrate;

(2) the step of forming a first polycrystal portion by irradiating thefirst region of the amorphous silicon film with a laser of which thewavelength is not less than 390 nm and not more than 640 nm; and

(3) the step of forming a second polycrystal portion that contacts thefirst polycrystal portion by irradiating the second region of theamorphous silicon film and a portion of the first polycrystal portionthat contacts the second region with a laser of which the wavelength isnot less than 390 nm and not more than 640 nm.

According to a process for a polycrystal silicon film provided with suchsteps, first, in the step shown in (2), the first region of theamorphous silicon film is irradiated with a laser so as to form thefirst polycrystal portion and, after that, the second region of theamorphous silicon film and a portion of the region of the firstpolycrystal portion are irradiated with a laser so as to form the secondpolycrystal portion that contacts the first polycrystal portion and,thereby, the first polycrystal portion and the second polycrystalportion can be formed. As a result, the amorphous silicon film can bepolycrystallized over a large area so that a polycrystal silicon filmhaving a large area can be formed.

Furthermore, the laser, of which the wavelength is in the abovedescribed range, has a large absorption coefficient with respect toamorphous silicon while the absorption ratio with respect to polycrystalsilicon is small and, therefore, the amorphous silicon is converted to apolycrystal silicon through the first irradiation and, then, nocharacteristics are changed due to the second irradiation in the portionthat is twice irradiated with the laser. Therefore, there is nodifference in the characteristics between the portion irradiated withthe laser once and the portion irradiated with the laser twice so that ahigh quality polycrystal silicon film can be provided.

The reason why the wavelength of the laser is not less than 390 nm isthat in the case that the wavelength of the laser is less than 390 nm,the absorption ratio of the polycrystal silicon film exceeds 60% of theabsorption ratio of the amorphous silicon film so that thecharacteristics of the polycrystal silicon film are changed through thesecond irradiation of the laser, which is not desirable. In addition,the reason why the wavelength of the laser is not more than 640 nm isthat in the case that the wavelength of the laser exceeds 640 nm, theabsorption ratio of the amorphous silicon becomes 10%, or less, so thatthe productivity is reduced.

In addition, the step of forming a first polycrystal portion preferablyincludes the formation of a first polycrystal portion that extends inone direction by scanning the laser on the amorphous silicon film. Thestep of forming a second polycrystal portion includes the formation of asecond polycrystal portion that extends along the first polycrystalportion by scanning the laser in the direction wherein the firstpolycrystal portion extends.

In this case, since both the first polycrystal portion and the secondpolycrystal portion are formed by laser scanning, the first and thesecond polycrystal portions can be formed so as to extend in apredetermined direction. Thereby, the first and the second polycrystalportions can be formed on a substrate of an even larger area.

In addition, the step of forming a first polycrystal portion preferablyincludes the irradiation of the amorphous silicon film with a firstlaser of which the wavelength is not less than 390 nm and not more than640 nm from a first laser light source. The step of forming a secondpolycrystal portion includes the irradiation of the amorphous siliconfilm with a second laser of which the wavelength is not less than 390 nmand not more than 640 nm from a second laser light source. In this case,the first polycrystal portion is formed through the irradiation with thelaser from the first laser light source while the second polycrystalportion is formed through the irradiation with the laser from the secondlaser light source and, therefore, the first and the second polycrystalportions can be formed almost simultaneously. Therefore, theproductivity of a polycrystal silicon film is increased and a largelaser output can be gained in a stable manner.

In addition, the above described laser preferably includes at least onetype selected from among a group consisting of the second harmonics ofan Nd: YAG laser, the second harmonics of an Nd: YVO₄ laser, the secondharmonics of an Nd: YLF laser, the second harmonics of an Nd: glasslaser, the second harmonics of a Yb: YAG laser, the second harmonics ofa Yb: glass laser, an Ar ion laser, the second harmonics of a Ti:sapphire laser and a Dye laser. In this case, these lasers can generatea laser of which the wavelength is not less than 390 nm and not morethan 640 nm.

In addition, the process for a polycrystal silicon film includes theirradiation with a laser from the second laser light source at aninterval of a predetermined period of time after the irradiation with alaser from the first laser light source. In this case, since the laserfrom the second laser light source can perform irradiation after thelaser from the first laser light source has performed irradiation, thefirst polycrystal portion and the second polycrystal portion can beformed in sequence. As a result, the production efficiency is furtherincreased.

A production unit for a polycrystal silicon film according to thisinvention is provided with an oscillation means, an irradiation means, ashifting means and a control means. The oscillation means allows alaser, of which the wavelength is not less than 390 nm and not more than640 nm to oscillate. The irradiation means irradiates an amorphoussilicon film formed on a substrate with a laser that is allowed tooscillator by the oscillation means. The shifting means shifts thesubstrate relative to the irradiation means. The control means controlsthe shifting means so that the laser carries out scanning so as to forma first polycrystal portion by irradiating the amorphous silicon filmwith the laser of which the wavelength is not less than 390 nm and notmore than 640 nm and so as to form a second polycrystal portion thatcontacts the first polycrystal portion by irradiating a portion of theamorphous silicon film that partially overlaps the first polycrystalportion with the laser of which the wavelength is not less than 390 nmand not more than 640 nm.

In such a production unit for a polycrystal silicon film, the controlmeans irradiates the amorphous silicon film with the laser so as to formthe first polycrystal portion and irradiates the portion that partiallyoverlaps the first polycrystal portion with the laser so as to form thesecond polycrystal portion and, therefore, the first and secondpolycrystal portions can be formed over a broad area. Therefore, apolycrystal silicon film with a broad area can be provided. Furthermore,a laser of which the wavelength is in the above described range has alarge absorption ratio with respect to amorphous silicon and has a smallabsorption ratio with respect to polycrystal silicon so that the portionthat is irradiated twice does not change. Therefore, a high qualitypolycrystal silicon film can be provided.

The reason why the wavelength of the laser is not less than 390 nm isthat in the case that the wavelength of the laser is less than 390 nm,the absorption ratio of the polycrystal silicon film exceeds 60% of theabsorption ratio of the amorphous silicon film so that thecharacteristics of the polycrystal silicon film are changed through thesecond irradiation of the laser, which is not desirable. In addition,the reason why the wavelength of the laser is not more than 640 nm isthat in the case that the wavelength of the laser exceeds 640 nm, theabsorption ratio of the amorphous silicon becomes 10%, or less, so thatthe productivity is reduced.

In addition, the irradiation means preferably includes the firstirradiation means and the second irradiation means. The amorphoussilicon film is irradiated with a portion of the laser that is allowedto oscillate by the oscillation means via the first irradiation means.The amorphous silicon film is irradiated with another portion of thelaser that is allowed to oscillate by the oscillation means via thesecond irradiation means. In this case, the amorphous silicon film canbe irradiated with the laser that is allowed to oscillate by oneoscillation means via the first irradiation means and the secondirradiation means and, therefore, the unit can be manufactured at lowcost.

In addition, the control means preferably controls the first and secondirradiation means, the oscillation means and the shifting means so thatthe second irradiation means performs the irradiation with the laser atan interval of a predetermined period of time after the firstirradiation means has performed the irradiation with the laser. In thiscase, the second irradiation means performs the irradiation with thelaser after the first irradiation means has performed the irradiationwith the laser and, therefore, the first polycrystal portion and thesecond polycrystal portion can be effectively produced. Therefore, apolycrystal silicon film production unit with a high productivity can beprovided.

In addition, the irradiation means preferably includes a firstirradiation means and a second irradiation means while the oscillationmeans includes a first oscillation means and a second oscillation means.The amorphous silicon film is irradiated with a laser that is allowed tooscillate by the first oscillation means via the first irradiationmeans. The amorphous silicon film is irradiated with a laser that isallowed to oscillate by the second oscillation means via the secondirradiation means. In this case, since two oscillation meansrespectively allow lasers to oscillate, lasers of which the outputs aresufficiently large can be allowed to oscillate in a stable manner sothat the first and the second polycrystal portions can be effectivelyproduced.

In addition, the control means preferably controls the first and secondirradiation means, the first and second oscillation means as well as theshifting means so that the second irradiation means performs theirradiation with the laser at an interval of a period of time after thefirst irradiation means has performed the irradiation with the laser. Inthis case, the second irradiation means can perform the irradiation withthe laser after the first irradiation means has performed theirradiation with the laser and, therefore, a polycrystal silicon filmcan be efficiently produced.

In addition, the oscillation means preferably allows the oscillation ofa laser that includes at least one type selected from among a groupconsisting of the second harmonics of an Nd: YAG laser, the secondharmonics of an Nd: YVO₄ laser, the second harmonics of an Nd: YLFlaser, the second harmonics of an Nd: glass laser, the second harmonicsof a Yb: YAG laser, the second harmonics of a Yb: glass laser, an Ar ionlaser, the second harmonics of a Ti: sapphire laser and a Dye laser.

A process for a semiconductor device according to this invention isprovided with the step of forming, on a substrate, an amorphous siliconfilm having a first region and a second region that contacts this firstregion and the step of forming a polycrystal silicon film by irradiatingthe amorphous silicon film with a laser. The step of forming apolycrystal silicon film includes the step of forming a firstpolycrystal portion by irradiating the first region of the amorphoussilicon film with a laser of which the wavelength is not less than 390nm and not more than 640 nm and the step of forming a second polycrystalportion that contacts the first polycrystal portion by irradiating thesecond region as well as the portion of the first polycrystal portionthat contacts the second region of the amorphous silicon film with alaser of which the wavelength is not less than 390 nm and not more than640 nm.

According to such a process, the first region of the amorphous siliconfilm is irradiated with the laser so as to form the first polycrystalportion and, after that, the second region and the portion of the regionof the first polycrystal portion of the amorphous silicon film areirradiated with the laser so as to form the second polycrystal portionthat contacts the first polycrystal portion and, therefore, the firstpolycrystal portion and the second polycrystal portion can be formed. Asa result, the amorphous silicon film can be polycrystallized over abroad area so that a polycrystal silicon film of a large area can beformed.

Furthermore, the laser, of which the wavelength is in the abovedescribed range, has a large absorption coefficient with respect toamorphous silicon while the absorption ratio with respect to polycrystalsilicon is small and, therefore, the amorphous silicon is converted to apolycrystal silicon through the first irradiation and, then, nocharacteristics are changed due to the second irradiation in the portionthat is twice irradiated with the laser. Therefore, there is nodifference in the characteristics between the portion irradiated withthe laser once and the portion irradiated with the laser twice so that ahigh quality polycrystal silicon film can be provided.

The reason why the wavelength of the laser is not less than 390 nm isthat in the case that the wavelength of the laser is less than 390 nm,the absorption ratio of the polycrystal silicon film exceeds 60% of theabsorption ratio of the amorphous silicon film so that thecharacteristics of the polycrystal silicon film are changed through thesecond irradiation of the laser, which is not desirable. In addition,the reason why the wavelength of the laser is not more than 640 nm isthat in the case that the wavelength of the laser exceeds 640 nm, theabsorption ratio of the amorphous silicon becomes 10%, or less, so thatthe productivity is reduced.

In addition, a semiconductor device according to this invention uses apolycrystal silicon film produced by means of the above describedprocess as an active region. In this case, since a polycrystal siliconfilm of which the mobility is large and of which the area is large isused as an active region, a semiconductor device of which the area islarge and that is of high performance can be provided

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view showing the first step of a process fora polycrystal silicon film according to a first embodiment of thisinvention;

FIG. 2 is a cross sectional view showing the second step of a processfor a polycrystal silicon film according to the first embodiment of thisinvention;

FIG. 3 is a view showing the detail of the step shown in FIG. 2;

FIG. 4 is a cross sectional view showing the third step of a process fora polycrystal silicon film according to the first embodiment of thisinvention;

FIG. 5 is a perspective view showing the detail: of the step shown inFIG. 4;

FIG. 6 is a cross sectional view showing the third step of a process fora polycrystal silicon film according to the first embodiment of thisinvention;

FIG. 7 is a graph showing the relationship between the wavelength of alaser and the absorption ratio with respect to an amorphous silicon filmand a polycrystal silicon film;

FIG. 8 is a graph showing the relationship between the film thickness ofa silicon film and the absorption ratio with respect to an amorphoussilicon film and a polycrystal silicon film;

FIG. 9 is a graph showing the relationship between irradiation positionand mobility in polycrystalline silicon produced according to thisinvention;

FIG. 10 is a graph showing the relationship between irradiation positionof a laser and threshold potential in a polycrystalline silicon filmproduced according to this invention;

FIG. 11 is a graph showing the relationship between the film thicknessand the absorption ratio in an amorphous silicon film and a polycrystalsilicon film in the case that a conventional excimer laser is used;

FIG. 12 is a graph showing the laser energy density and the mobility inthe case that a polycrystal silicon film is produced using an excimerlaser;

FIG. 13 is a perspective view showing the step of producing apolycrystal silicon film according to a second embodiment of thisinvention;

FIG. 14 is a view showing one example of a laser scanning method in FIG.13;

FIG. 15 is a view showing another example of a laser scanning method inFIG. 13;

FIG. 16 is a view showing linear beams that overlap;

FIG. 17 is a view showing a process for a polycrystal silicon filmaccording to a third embodiment of this invention;

FIG. 18 is a cross sectional view showing the first step of a processfor a polycrystal silicon film according to a prior art;

FIG. 19 is a cross sectional view showing the second step of a processfor a polycrystal silicon film according to the prior art; and

FIG. 20 is a cross sectional view showing the third step of a processfor a polycrystal silicon film according to the prior art.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

FIGS. 1 to 6 are views showing a process for a polycrystal silicon filmaccording to a first embodiment of this invention. Referring to FIG. 1,a silicon oxide film 32 is formed on a glass substrate 31 by means of,for example, a CVD method. An amorphous silicon film 33 is formed onsilicon oxide film 32 by means of a CVD method. This amorphous siliconfilm 33 has a first region 33 a and a second region 33 b that contactsthis first region 33 a.

Referring to FIGS. 2 and 3, first region 33 a of amorphous silicon film33 is irradiated with the second harmonics laser (wavelength 532 nm) ofNd: YAG of the Q switch. Thereby, a portion irradiated with a laser 35is polycrystallized so as to form a first polycrystal portion 34 a. Atthis time, a unit, as shown in FIG. 3, is used for the irradiation bythe laser.

Referring to FIG. 3, a polycrystal silicon film production unit 100 isprovided with a laser oscillator 120 as an oscillation means foroscillating a laser of which the wavelength is not less than 390 nm andnot more than 640 nm, an irradiation means 110 for irradiating anamorphous silicon film formed on a substrate with a laser oscillatedfrom laser oscillator 120, a shifting means 130 for shifting a substraterelative to the irradiation means and a control means 140 forcontrolling the shifting means that forms a first polycrystal portion byirradiating an amorphous silicon film with a laser, of which thewavelength is not less than 390 nm and not more than 640 nm, and thatforms a second polycrystal portion that contacts the first polycrystalportion by irradiating the amorphous silicon film with a laser, of whichthe wavelength is not less than 390 nm and not more than 640 nm, so thatthe irradiation overlaps a portion of the first polycrystal portion.

Laser oscillator 120 is a Q switch Nd: YAG laser second harmonicsoscillator and oscillates a laser so that amorphous silicon film 33formed on glass substrate 31 is irradiated with this laser viairradiation means 110. Here, the silicon oxide film between glasssubstrate 31 and amorphous silicon film 33 is omitted in FIG. 3.

Irradiation means 110 is formed of a mirror 111 and a beam formationoptical system 112. Beam formation optical system 112 forms the laserbeam emitted from laser oscillator 120 into a predetermined form. Then,the laser emitted from beam formation optical system 112 is reflected bymirror 111 so as to irradiate amorphous silicon film 33. Beam formationoptical system 112 and mirror 111 are both positioned above amorphoussilicon film 33.

Shifting means 130 is formed of a movable stage 131 and a driving motor132 for driving movable stage 131. Movable stage 131 supports glasssubstrate 31 and is allowed to shift relative to laser oscillator 120and irradiation means 110. Therefore, when movable stage 131 moves,glass substrate 31 and amorphous silicon film 33 that are mountedthereon also move.

Movable stage 131 is connected to driving motor 132 so that drivingmotor 132 drives movable stage 131. Here, movable stage 131 is allowedto shift on a predetermined plane in all directions.

Control means 140 is connected to driving motor 132 and laser oscillator120. Control means 140 sends a signal to driving motor 132 so that itdrives movable stage 131 at a predetermined time. Driving motor 132 thathas received this signal shifts movable stage 131 in a predetermineddirection. In addition, control means 140 sends a signal to laseroscillator 120 and allows laser oscillator 120 to oscillate a laser.

Control means 140 sends a signal to laser oscillator 120 by using such aunit. Laser oscillator 120 oscillates a laser so as to irradiate firstregion 33 a of amorphous silicon film 33 with this laser via beamformation optical system 112 and mirror 111. Under this condition,control means 140 sends a signal to driving motor 132 so that drivingmotor 132 shifts movable stage 131 in the direction shown by arrow 131a. Thereby, the portion irradiated with the laser is crystallized so asto form first polycrystal portion 34 a.

Referring to FIGS. 4 and 5, after first polycrystal portion 34 a hasbeen formed, laser oscillator 120 stops oscillating the laser. Movablestage 131 is used to shift amorphous silicon film 33 so that firstpolycrystal portion 34 a and second region 33 b are irradiated withlaser 35 in a line form. Under this condition, laser 35 is allowed toscan and, thereby, second polycrystal portion 34 b is formed.

That is to say, the step of forming first polycrystal portion 34 aincludes the formation of first polycrystal portion 34 a that extends inone direction by allowing laser 35 to scan over amorphous silicon film33 while the step of forming second polycrystal portion 34 b includesthe formation of second polycrystal portion 34 b that extends alongfirst polycrystal portion 34 a by allowing laser 35 to scan in thedirection in which first polycrystal portion 34 a extends.

By repeating this operation, the major portion of amorphous silicon film33 is polycrystallized so as to form polycrystal silicon film 34.

Referring to FIG. 6, polycrystal silicon film 34 is patterned, whileleaving out a predetermined portion as polycrystal silicon film 34, soas to form active regions 39 a and 39 b. Next, a silicon oxide film isformed on polycrystal silicon film 34. A metal film made of a lowresistance metal such as Ta, Cr or Al is formed on the silicon oxidefilm. By patterning the metal film and the silicon oxide film into apredetermined form, gate insulating films 36 a and 36 b, as well as gateelectrodes 37 a and 37 b, are formed. After that, gate electrodes 37 aand 37 b are used as a mask to form sources and drains in a self-alignedmanner by means of an ion doping method. Thereby, a thin film transistoris completed.

That is to say, in the process for a semiconductor device according tothis invention, an amorphous silicon film is formed on glass substrate31 having first region 33 a and second region 33 b that is connected tothis first region 33 a on glass substrate 31. Next, amorphous siliconfilm 33 is irradiated with a laser so as to form polycrystal siliconfilm 34. The step of forming polycrystal silicon film 34 includes thesteps wherein, first region 33 a of amorphous silicon film 33 isirradiated with a laser of which the wavelength is not less than 390 nmand not more than 640 nm so as to form first polycrystal portion 34 aand, next, wherein second region 33 b of the amorphous silicon film anda region of a portion of first polycrystal portion. 34 a that contactssecond region 33 b are irradiated with a laser of which the wavelengthis not less than 390 nm and not more than 640 nm so as to form secondpolycrystal portion 34 b that contacts first polycrystal portion 34 a.In addition, the semiconductor device thus gained uses the polycrystalsilicon film fabricated in the above described steps as active regions39 a and 39 b.

FIG. 7 is a graph showing the relationships of the wavelength of a laserand the absorption ratio in an amorphous silicon film and in apolycrystal silicon film. Referring to FIG. 7, the absorption ratio of alaser in the amorphous silicon film and in the polycrystal silicon filmchanges variously depending on the wavelength thereof. Since thewavelength of the laser is not less than 390 nm in the presentinvention, the absorption ratio of the polycrystal silicon film is 60%,or less, of the absorption ratio of the amorphous silicon film.Therefore, in the case that polycrystal silicon has been formed throughlaser irradiation of amorphous silicon, such polycrystal silicon doesnot absorb the energy of the laser even when the polycrystal silicon isirradiated with the laser. As a result, the characteristics of thepolycrystal silicon do not vary so that almost the same characteristicscan be gained throughout the entirety of the polycrystal silicon film.

In addition, since the wavelength of the laser is not more than 640 nm,the absorption ratio in the amorphous silicon film becomes 10%, orgreater. As a result, it becomes easy for the amorphous silicon toabsorb the heat from the laser so that the amorphous silicon can beeasily polycrystallized.

Here, it is preferable for the wavelength to be not less than 500 nm andnot more than 550 nm because the difference in the absorption ratios ofthe amorphous silicon film and the polycrystal silicon film becomesgreater. It is more preferable for the wavelength to be not less than520 nm and not more than 550 nm because the difference in the absorptionratios of the amorphous silicon film and the polycrystal silicon filmbecomes particularly great.

FIG. 8 is a graph showing the relationships between the silicon filmthickness and the absorption ratio with respect to the laser (secondharmonics of Nd: YAG (wavelength λ=532 nm)) used in this invention. Theabsorption ratio of the polycrystal silicon film is smaller than theabsorption ratio of the amorphous silicon film in the case that thethickness of the silicon film is set at a variety of values with respectto the laser used in this invention.

In addition, n channel type and p channel type transistors in thestructure shown in FIG. 6 are fabricated and the mobility and thethreshold potential in these transistors are shown in FIGS. 9 and 10.

Referring to FIG. 9, the portion surrounded by solid line 201 indicatesthe portion that is twice irradiated with the laser. It can beunderstood from FIG. 9 that the mobility is maintained at anapproximately constant level in the case that either an n channel typetransistor or a p channel type transistor is formed. In addition, it canbe understood that the mobility in the portion that is twice irradiatedwith the laser is approximately the same as that in the other portions.

Referring to FIG. 10, the portion surrounded by solid line 202 is theportion that is twice irradiated with the laser. It can be understoodfrom FIG. 10 that the threshold voltage of either the n channel typetransistor or the p channel type transistor is approximately the same inany position. In addition, it can be understood that the thresholdpotential is approximately the same in the portion that is twiceirradiated with the laser and in the portion that is irradiated with thelaser only once.

The wavelength of the laser is in an optimal range in the above manneraccording to the present invention and, therefore, a high qualitysemiconductor device can be provided wherein the mobility and thethreshold potential are constant in both the portion that is onceirradiated with the laser as well as in the portion that is twiceirradiated with the laser.

That is to say, in the case that a thin film transistor is formed, thethickness of the silicon film is conventionally 100 nm, or less, and inthis region the absorption ratios of the amorphous silicon film and ofthe polycrystal silicon film differ greatly wherein the absorption ratioof the polycrystal silicon film is smaller than the absorption ratio ofthe amorphous silicon film. As a result, when a polycrystal silicon filmis irradiated with a laser having an irradiation energy density that isoptimal for amorphous silicon film, the energy absorbed by thepolycrystal silicon film is too small to cause the melting of thepolycrystal silicon film. That is to say, only the amorphous siliconfilm portion selectively receives the laser heat processing and,therefore, no difference is caused in the characteristics of the portionthat receives laser heat processing twice and the portion that onlyreceives laser heat processing once so that a polycrystal silicon filmcan be formed of which the characteristics are uniform throughout theregion of the substrate. In addition, the same effects can be gained inthe case that a polycrystal silicon film that has many crystal defectsand of which the absorption high is used as a film that is irradiatedwith the laser.

FIG. 11 is a graph showing the relationships between the film thicknessand the absorption ratio of a conventional amorphous silicon film and ofa polycrystal silicon film fabricated by using an excimer laser. It canbe understood from FIG. 11 that the absorption ratios of the polycrystalsilicon film and of the amorphous silicon film are approximately thesame. Even in the case that such an amorphous silicon film has beenconverted to a polycrystal silicon film by being irradiated once with alaser, the polycrystal silicon film is again irradiated with a laserafterwards and, thereby, the polycrystal silicon film absorbs the energyof the laser. Thereby, the polycrystal silicon film is again melted sothat the characteristics of the polycrystal silicon film change.Therefore, the portion that is once irradiated with the laser and theportion that is twice irradiated with the laser differ in thecharacteristics of the polycrystal silicon film and a polycrystalsilicon film that has uniform characteristics throughout the film cannotbe gained.

That is to say, as shown in FIG. 11, in the case that the amorphoussilicon film and the polycrystal silicon film are irradiated with a KrFexcimer laser beam (wavelength: 248 nm), the difference in absorptionratios of the amorphous silicon film and the polycrystal silicon film isapproximately 7%. At the time of laser heat processing of the amorphoussilicon film, the irradiation energy density is set at the optimal valueof the amorphous silicon film.

FIG. 12 is a graph showing the relationships between the laser energydensity and the mobility of the n channel type transistor at the timewhen the polycrystal silicon film, shown in FIG. 11, is fabricated. Asshown in FIG. 12, as for heat processing by means of an excimer laser,the permissible range of the optimal value of the irradiation energydensity is very narrow and, therefore, when the absorption ratios differby 7% it becomes a problem. That is to say, after the polycrystalsilicon film portion has been once melted through the irradiation bymeans of the excimer laser, recrystallization growth occurs and sincethe irradiation energy density is outside of the permissible range ofthe optimal value, the region that has undergone a second laserirradiation is converted to a polycrystal silicon film having poorcharacteristics.

That is to say, the portion that is once irradiated with a laser and theportion that is twice irradiated with a laser in a conventionalpolycrystal silicon film differ in laser energy density. Therefore, themobility of the n channel type transistor differs so that uniformcharacteristics cannot be gained throughout the polycrystal siliconfilm. As a result, a thin film transistor having the desiredcharacteristics cannot be gained.

Second Embodiment

FIG. 13 is a perspective view showing a process for a polycrystalsilicon film according to a second embodiment of this invention. Theirradiation means of a polycrystal silicon film production unit 180,shown in FIG. 13, differs from that of the polycrystal silicon filmproduction unit 100. That is to say, in the polycrystal silicon filmproduction unit 180, shown in FIG. 13, a first irradiation means 110 a,a second irradiation means 110 b and a third irradiation means 110 cexist as the irradiation means. First, second and third irradiationmeans 110 a, 110 b and 110 c are, respectively, formed of a mirror 111and a beam formation optical system 112. Mirror 111 and beam formationoptical system 112 are similar to those shown in FIG. 3. A laseroscillator 120 a, as the first oscillation means, a laser oscillator 120b as the second oscillation means and a laser oscillator 120 c as thethird oscillation means are connected to respective beam formationoptical systems 112. Laser oscillators 120 a, 120 b and 120 c are,respectively, Q switch Nd: YAG laser second harmonics oscillators. Lightemitted from, respectively, laser oscillators 120 a, 120 b and 120 cirradiates the beam formation optical system. In addition, laseroscillators 120 a, 120 b and 120 c are respectively connected to controlmeans 140.

A laser oscillated from laser oscillator 120 a irradiates amorphoussilicon film 33 via first irradiation means 110 a. A laser oscillatedfrom laser oscillator 120 b irradiates amorphous silicon film 33 viasecond irradiation means 110 b. Control means 140 controls first andsecond irradiation means 110 a and 110 b, laser oscillators 120 a and120 b as well as shifting means 130 so that second irradiation means 110b emits a laser 35 b after a predetermined period of time has elapsedsince first irradiation means 110 a has emitted a laser 35 a.

That is to say, as shown in FIG. 13, amorphous silicon film 33 isirradiated with lasers 35 a, 35 b and 35 c and, under this condition,shifting means 130 allows glass substrate 31 to shift in the directionshown by arrow 131 a and, thereby, the surface of amorphous silicon film33 is irradiated with lasers 35 a, 35 b and 35 c so that a polycrystalsilicon film can be formed on amorphous silicon film 33.

FIG. 14 is a view showing the appearance of a laser irradiating theamorphous silicon film. Referring to FIG. 14, the positions of thelasers are arranged so that, first, laser 35 a becomes the head and isfollowed by laser 35 b which is, in turn, followed by laser 35 crelative to the shifting direction of the movable stage shown by arrow131 a.

In addition, referring to FIG. 15, lasers 35 a and 35 c are arranged soas to be positioned in front toward the direction of progression shownby arrow 131 a while laser 35 b may be arranged so as to be positionedon the rear side. In these methods the positions of respective beams inline forms are parallel to each other and are staggered and, inaddition, the edges of the beams in line forms are slightly overlappedwith the edges of the adjacent beams so that the heat processed tracesof the adjacent beams overlap each other. A plurality of beams in lineforms of such a configuration irradiate, simultaneously or in a timestaggered manner, the stage while it is being scanned.

Furthermore, as shown in FIG. 16, the beams are arranged so thatrespective lasers 35 a, 35 b and 35 c overlap. In FIG. 16, therespective beams in line forms are connected so as to form a beam in aone line form as shown in the figure. As for the laser irradiation, eachof the laser beams irradiates the stage in a time staggered manner sothat the adjoining two beams do not simultaneously carry out irradiationwhile the stage is being scanned.

A semiconductor device that has a polycrystal silicon film can bemanufactured by using such a polycrystal silicon film production unit180 with the same method as the method of the first embodiment.Furthermore, three laser oscillators are used in this unit and,therefore, throughput is increased and a polycrystal silicon film of abroad area can be effectively produced.

Third Embodiment

FIG. 17 is a perspective view of a production unit for a polycrystalsilicon film according to a third embodiment of this invention.Referring to FIG. 17, a polycrystal silicon film production unit 190according to the third embodiment differs from that of other embodimentsin the point that a laser oscillated from one laser oscillator 420 isused by three irradiation means. That is to say, the irradiation meanshas a first irradiation means 210 a, a second irradiation means 210 band a third irradiation means 210 c. Irradiation means 210 a, 210 b and210 c have, respectively, a beam formation optical system 112 and amirror 111 that are the same as those in the first embodiment. Themirror 114 reflects a laser that is oscillated by laser oscillator 420of which the wavelength is not less than 390 nm and not more than 640 nmso that amorphous silicon film 33 is irradiated with this laser in theform of lasers 35 a, 35 b and 35 c via beam formation optical systems112 and mirrors 111. That is to say, in this polycrystal silicon filmproduction unit 190 the irradiation means includes first irradiationmeans 210 a and second irradiation means 210 b. Amorphous silicon film33 is irradiated with a portion of the laser oscillated from laseroscillator 420 via first irradiation means 210 a. Amorphous silicon film33 is irradiated with another portion of the laser oscillated from laseroscillator 420 via second irradiation means 210 b. In addition, controlmeans 140 controls first and second irradiation means 210 a and 210 b,laser oscillator 420 as well as shifting means 130 so that secondirradiation means 210 b carries out a laser irradiation after apredetermined period of time has passed since first irradiation means210 a has carried out a laser irradiation.

Nd: YAG second harmonics (wavelength: 532 nm) lasers emitted from laseroscillator 420 are formed to have beam profiles in line forms by meansof beam formation optical systems 112 that correspond to respectivelasers.

Such a method also has the same effect as in the first embodiment.Furthermore, the unit can be formed of one laser oscillator 420 so thatthe cost of the unit can be reduced.

In addition to the above description of the embodiments of thisinvention, it is possible to modify the embodiments shown herein in avariety of manners. First, a laser irradiation method as shown in FIGS.14 to 16 of the second embodiment can be used in the unit shown in FIG.17. Furthermore, in addition to the means for oscillating the secondharmonics of an Nd: YAG laser that is shown as a laser oscillator, otherlaser oscillators such as laser oscillators that oscillate the secondharmonics of an Nd: YVO₄ laser, the second harmonics of an Nd: YLFlaser, the second harmonics of an Nd: glass laser, the second harmonicsof a Yb: YAG laser, the second harmonics of a Yb: glass laser, an Ar ionlaser, the second harmonics of a Ti: sapphire laser or a Dye laser maybe used.

Industrial Applicability

This invention can be utilized in the field of a process for thin filmtransistors that are used in a liquid crystal display.

1. A process for making a polycrystalline silicon film comprising:forming an amorphous silicon film having a first region and a secondregion that partially overlaps the first region, on a substrate;condensing a laser beam having a wavelength not shorter than 390 nm andnot loner than 640 nm to form a linear laser beam; forming a firstpolycrystalline portion by scanning the first region of the amorphoussilicon film with the linear laser beam in a single scan direction sothat the first polycrystalline portion extends in the scan direction;and forming a second polycrystalline portion that partially overlaps thefirst polycrystalline portion by scanning the second region of theamorphous silicon film and the part of the first region that overlapsthe second region with the linear laser beam so that the secondpolycrystalline portion extends in the scan direction and issubstantially parallel to the first polycrystalline portion.
 2. Theprocess for making a polycrystalline silicon film according to claim 1,wherein forming the first polycrystalline portion includes irradiatingthe first region of the amorphous silicon film with a first linear laserbeam having a wavelength not shorter than 390 nm and not longer than 640nm from a first laser light source and forming the secondpolycrystalline portion includes irradiating the first region of theamorphous silicon film with a second linear laser beam having awavelength not shorter than 390 nm and not longer than 640 nm from asecond laser light source.
 3. The process for making a polycrystallinesilicon film according to claim 1, wherein the linear laser beam isselected from the group consisting of second harmonic light of an Nd:YAGlaser, second harmonic light of an Nd:YVO₄ laser, second harmonic lightof an Nd:YLF laser, second harmonic light of an Nd:glass laser, secondharmonic light of a Yb:YAG laser, second harmonic light of a Yb:glasslaser, light of an Ar ion laser, second harmonic of a Ti:sapphire laser,and light of a dye laser.
 4. The process for making a polycrystallinesilicon film according to claim 2, including irradiating with the secondlinear laser beam from the second laser light source after a fixedperiod of time has elapsed after irradiation with the first linear laserbeam from the first laser light source.
 5. A production unit forproducing a polycrystalline silicon film comprising: oscillation meansfor producing a laser beam having a wavelength not shorter than 390 nmand not longer than 640 nm; irradiation means for condensing the laserbeam to form a linear laser beam and irradiating an amorphous siliconfilm on a substrate with the linear laser beam; shifting means forshifting the substrate relative to said irradiation means; and controlmeans for controlling said shifting means so the linear laser beam scansthe amorphous silicon film so that a first polycrystalline portion isformed by scanning a first region of the amorphous silicon film in ascan direction with the linear laser beam, that a second polycrystallineportion having a part overlapping said first polycrystalline portion isformed by scanning the part of the first polycrystalline silicon portionthat overlaps the second polycrystalline portion and a second region ofthe amorphous silicon film with the linear laser beam in the scandirection, and the first and second polycrystalline portions extend inthe scan direction and are substantially parallel to each other.
 6. Theproduction unit for producing a polycrystalline silicon film accordingto claim 5, wherein said irradiation means includes first irradiationmeans and second irradiation means; the amorphous silicon film isirradiated with a first portion of the linear laser beam from saidoscillation means via said first irradiation means; and the amorphoussilicon film is irradiated with a second portion of the linear laserbeam from said oscillation means via said second irradiation means. 7.The production unit for producing a polycrystalline silicon filmaccording to claim 6, wherein said control means controls said first andsecond irradiation means, said oscillation means, and said shiftingmeans so that said second irradiation means provides irradiation by asecond linear laser beam after a fixed period of time has elapsed sincesaid first irradiation means has provided irradiation by a first linearlaser beam.
 8. The production unit for producing a polycrystallinesilicon film according to claim 5, wherein said irradiation meansincludes first irradiation means and second irradiation means and saidoscillation means includes first oscillation means and secondoscillation means; the amorphous silicon film is irradiated with a firstlinear laser beam from said first oscillation means via said firstirradiation means; and the amorphous silicon film is irradiated with asecond linear laser beam from said second oscillation means via saidsecond irradiation means.
 9. The production unit for producing apolycrystalline silicon film according to claim 8, wherein said controlmeans controls said first and second irradiation means, said first andsecond oscillation means, and said shifting means so that said secondirradiation means provides irradiation by the second linear laser beamafter a fixed period of time has elapsed since said first irradiationmeans has provided irradiation by the first linear laser beam.
 10. Theproduction unit for producing a polycrystalline silicon film accordingto claim 5, wherein said oscillation means produces the linear laserbeam that includes at least one selected from the group consisting ofsecond harmonic light of an Nd:YAG laser, second harmonic light of anNd:YVO₄ laser, second harmonic light of an Nd:YLF laser, second harmoniclight of an Nd:glass laser, second harmonic light of a Yb:YAG laser,second harmonic light of a Yb:glass laser, light of an Ar ion laser,second harmonic light of a Ti:sapphire laser, and light of a dye laser.11. A process for producing a semiconductor device, comprising: formingan amorphous silicon film having a first region and a second region thatpartially overlaps the first region, on a substrate; condensing a laserbeam having a wavelength not shorter than 390 nm and not longer than 640nm to form a linear laser beam; and forming a polycrystalline siliconfilm by scanning the amorphous silicon film with the linear laser beam,wherein forming the polycrystalline silicon film includes forming afirst polycrystalline portion by scanning the first region of theamorphous silicon film with the linear laser beam in a single scandirection so that the first polycrystalline portion extends in the scandirection, and forming a second polycrystalline portion that partiallyoverlaps the first polycrystalline portion by scanning the second regionof the amorphous silicon film and the portion of the first region thatoverlaps the second region with the linear laser beam so that the secondpolycrystalline portion extends in the scan direction and issubstantially parallel to the first polycrystalline portion.
 12. Asemiconductor device wherein a polycrystalline silicon file producedaccording to claim 11 is used for active regions.